Progressive deformation and reorientation of fold axes in a ductile mylonite zone: the woodroffe thrust

‘l’ecfonophysics, 64 (1978) 285-320 0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

285

PROGRESSIVE DEFORMATION AND REORIENTATION OF FOLD AXES IN A DUCTILE MYLONITE ZONE: THE WOODROFFE THRUST

T.H. BELL Geology Department, (Submitted

James Cook University, Townsde,

QLD 4811 (Australia)

October 18, 1976; revised version accepted June 8,1977)

ABSTRACT Bell, T.H., 1978. Progressive deformation and reorientation of fold axes in a ductile mylonite zone: the Woodroffe thrust. Tectonophysics, 44: 285-320. The structural geometry of a mylonite zone (the Woodroffe thrust) and the country rock in its immediate vicinity is described. Mylonitic schistosity formed axial planar to folds in country rock foliation and contains a mineral elongation lineation which is constant in orientation. However, the fold axes (and associated intersection lineation) spread in orientation within the mylonitic schistosity but with a strong maximum parallel to the mineral elongation lineation. It is demonstrated that the fold axes formed initially at approximately 90“ to mineral elongation but rotated with increase in strain towards it. Where this phenomenon was homogeneous on a macroscopic scaIe, rotation of large blocks of country rock across zones of mylonitization accompanied reorientation of fold axes within the mylonite. The controversy of progressive simple versus progressive pure shear for mylonite zones is discussed in the light of recent fabric and other evidence, It is concluded that the inhomogenoous forms of both progressive pure shear and progressive simple shear played a part and that the former dominated initially but gradually gave way to the latter until brittle rupture with large simple-shear displacements on a zone lubricated by the formation of pseudotachylite, brought granulite over ~phibolite facies rocks.

INTRODUCTION

Two significant problems in structural geology which in recent years have received considerable attention are: (1) The problem of parallelism of mineral elongation lineation to fold axes (Johnson, 1968; Bryant and Reed, 1969; Sander-son, 1973; Escher and Watterson, 1974; Roberts and Sanderson, 1973; Nicolas and Boudier, 1975; Hobbs et al., 1976). (2) The problem of simple- versus pure-shear strain histories or combination thereof in mylonite or shear zones (Johnson, 1967; Ramsay and Graham, 1970; McLeish, 1971; Escher and Watterson, 1974; Escher et al., 1975; Wilkinson et al., 1975; Nicolas and Boudier, 1975).

286

This paper describes a ductile mylonite zone ity formed axial planar to folds in country rock schistosity the axes of these folds lie at variable west plunging mineral elongation lineation. The relationships to the above problems is discussed. ~EOL~~~ICAL

in which mylonitic schistosschistosity. In the mylonitic angles or parallel to a southrelevance of these and other

SETTING

The rocks described in this paper crop out along the Woodroffe thrust (Major et al., 1967) north of Amata in the Musgrave Ranges (Fig. la). At this point the thrust strikes approximately north-south and dips at about thirty degrees towards the west (Fig. lb). On the western side, and above the thrust granulite-facies gneisses crop out, whereas to the east and below the thrust amph~bolite-facies gneisses occur. The country rocks on either side of the mylonite zone are acid gneisses with similar mineralogy and chemistry except for differences related their metamorphic grade. These differences are: (a) two pyroxenes versus hornblende plus biotite; and (b) lower water content and rubidium/strontium ratio on the granulite-facies side. They are considered to be diagnostic for distinguishing between granulite and amphibolite-facies rocks in this and other regions (c,f. Lambert and Heier, 1968; Collerson, 1972). The relationship between the country rocks on either side of the Woodroffe thrust is not known. They are structurally different (Table I, see later) and although the order of deformation events on each side can be determined, a positive correlation across the thrust cannot be made. The major penetrative schistosity in the granulite-facies rocks is the first deformation event recognizable above the thrust. However, in the amphibolite facies rocks the pervasive schistosity is due to a second (or later) deformation event. Hence schistosity and lineation subscripts used for these rocks in this paper have time significance only within their respective sides of the thrust and imply no correlation across the thrust. GEOMETRIC

ANALYSIS

The mylonites The Woodroffe thrust consists of a number of mylonite zones which anastomose around ellipsoidal blocks of relatively undeformed country rock. This anastomosing character occurs on all scales and the blocks of country rock vary in size from less than a millimeter to half a kilometer in their smallest dimension (Figs. 2 and 4). The blocks of country rock are generally remains of amphibolite-facies gneiss from below the mylonite zone, except within the upper 250 meters of the thrust zone where there are remains of what was originally granulite-facies gneiss. A band of pseudotachylite up to twenty meters thick occurs at the top

287 Saslc

and

ultra

basic

Intrusions Granulite

facles

TransItional amphibolrte

-v

Woodroffe

Thrust

Davenport

Shear

gnwss

granulate

Amphibollte

facles

gne~ss

N. T. ---S. A.

N MUSGRAVE

Alluwum

1:: : :]

Granullte

Amphlbohte

facles facies

gne~ss gne~ss

Mylomte

(Woodroffe

t-=_=i

Mylonlte

(Arparah

Subarea

Thrust) Fault

J‘

r’

Schlstoslty Mylonltic schlstoslt)

Fig

Fig. 1. (a) Locality and regional geology map of the Musgrave Range metamorphic belt. (b) Geological map of the area outlined in Fig. la, showing subarea locations and the broad relationships between the two different types of country-rock gneisses.

of the Woodroffe thrust zone, just below the first signs of mylonitization in the granulite-facies acid gneiss (Fig. 4). Above the pseudotachylite no folds of country-rock schistosity (Sr) with (or without) mylonitic schistosity (S,) axial plane were observed. (Refer to Table I for a summary of the structural history and symbols.) Although S, in subarea 5 (Fig. 9d) and S, in sub-

Fig. 2. Mylonitic schistosity tized country rock.

(S,)

anastomosing

around

ellipsoidal

pods of less myloni-

area 4 (Fig. 7j) are statistically subparallel, the map (Fig. 4) shows that there is significant strike variation across the country-rock-mylonite contact. Immediately below the pseudotachylite many isoclinal folds of Si with S, axial plane were observed. The contact of the amphibolite-facies acid gneiss with mylonite at the sole of the thrust zone (Fig. 2) shows that the mylonitic schistosity (S,) has formed axial planar to folds in the country-rock schistosity (Sz). These folds are open where S, is weak but become isoclinal where it is more intense (Fig. 3).

Structural

elements,

style and orientation

within the mylonite

The folds in country-rock schistosity with the mylonitic schistosity as axial plane are called either By * or ST folds (e.g. Fig. 5b) which were formed from granuliteor amphibolite-facies country rock, respectively (see Table I). The mylonitic schistosity (S,) is defined by alignment of flattened quartz and feldspar grains and aggregates, and (001) of micas. A very strong linea*B y is an abbreviation type setting.

s1 as used in Turner of BSm

and Weiss (1963,

p. 131)

for ease of

289

Fig. 3. A transition zone from country rock to mylonite which shows mylonitic schistosity S, forming axial planar to folds in country-rock schistosity Sz and gradually pervading the rock from the bottom to the top of the photograph.

be bent on one surface while on another 1 cm away it is straight. L,, however remains constant and L, appears to bend towards it (Fig. 6A, B).

Granulite-facies country rock Structural elements, style and orientation The granulite-facies country rock is structurally simple. It contains a strong penetrative schistosity (S,) defined by ellipsoidal quartz and feldspar grains and aligned (001) of rare biotite and is statistic~ly planar (Fig. 9d, f). The pronounced lineation (L,) lying in S, is due to aligned elongate quartz and feldspar grains and plots as a point maximum with the low contour intervals slightly spread out into S, (Fig. 9g, i). Rare folds showed S, axial planar to isoclinal intrafolial folds in a weak compositional layering (S,) defined mainly by magnetite content (Fig. 5e). The relationship of L, to Bi was not observed. The granulite-facies country rock is divided into two subareas by a zone of mylonite (subarea 6) along Arparah Creek (Figs. lb and 4). This mylonite zone displaces the Woodroffe thrust to the north (west side north) by about 100 metres, and the structural similarity between the subareas shows that there has been no rotation on this zone (Fig. 9d-k).

Amphibolite-facies

country rock

Structural elements, style and orientation These rocks are structur~ly more complex due to the presence of three

290 TABLE f Structural.

nomenclature

and chronology

East of the thrust + - amphibolite side

s Sl %2 LZ

B1Z sm I ‘me

Lm Bg, By ** and BE $j and B3m tVest of the thrust * _t kl 9 ‘1 ;f,

sm I Am

iayering - always parallel S1 or Sz schistosity - found rarefy in hinges of B$ folds schistosity - dominant outisde mylonite zone mineral elongation and streaking lineation in S2 hinges of BT folds folds in S/.&l with Sa as axial plane myionitic schistosity mineral elongation lineation in S, intersection Iineation in S, - parallel to hinges folds fohis in Sa, Sr **, and S, res ectivei with S, schistosity - axial plane to BzB and B,z folds folds in S2 and S, respectively with S, as axial

- parallel

of BF and By ** as axial plane plane

layering schistosity - dominant outside mylonite zone mineral elongation lineation in S, folds in Sc with S1 as axial plane mylonitic schistosity in subarea 6 - postdates Woodroffe thrust mylonization mineral elongation Iineation in S’, in subarea 6 - as above

____~._..___~_

* The thrust is defined as the pseudotachylite zone for the purpose of this table (cf. Pigs. 1 and 4). ** This refers to S1 in mylonitized remains of originally granulite-facies country rock which occupies the 250 metres immediately below the pseudotachylite zone (cf. Fig. 4).

tion (Am) on S, surfaces is due to their intersection with SI or Sz and is parallel to B!f or 23T fold axes. L, is often paraliel or subparallel to a mineral elongation lineation (L,,) due to aligned elongate quartz and feldspar grains and aggregates. However, in a number of instances (Fig. 6B) L, and I,,, are at a large angle to one another. L, may be bent through up to 180” in the plane of S, (Figs. 6, 7b, e, h, k) where I,,, is constant in orientation. fn plunging these cases, L,, is parallel to the 45, maximum concentration towards the southwest (cp. Fig. 10 with Fig. 7e, k). Another fold style occurs where the mylonitic schistosity is intrafolially folded and a weak to strong new mylonitic schistosity forms as axial plane. These folds are called Bg folds, and the sehistosity folded around them is identical in character to the mylonitic schistosity intrafolially enclosing them (Figs. 5c, lla). .I??, I?: (I,,) and I32 axes often bend through at least 180” in their own axial planes (i.e. their axial planes remain unfolded; Fig. 11B). This occurs on both the meso- and macroscopic scales as shown in Figs. 6a and 7. L, can

291-300

t.N

m ,---_

‘__-

Fig. 4. Structural map of the area outlined in Fig. la. Note in subarea 2 the change in the orientation of L, across the mylonite zone from subarea 1 to 3.

,,’

,/-

/’ I ,’

4’

,‘_

STRUCTURAL / - ,’

:OLOGY

BETWEEN

ARPARAH

AN[:

TOOGUL

MUSGRAVE

RANGES

r4ykmtic

. ...I __..

--f

Fold

Myionitr

Wwdrcftt

Thrust

Psrudo~tachylilr htyhY&t*

Arparah

PICUda&%hyi*tP Amphibolite Ookwte

GrolCpic

Creek

Dykc

Boundary

Fault

301

.1Em

Fig. 5. (a) Sketch of an outcrop which shows a number of Bi folds of layering parallel to schistosity (S II S1) which have a schistosity Sa aa axial plane. S2 itself has been folded by a large fold which has no axial-plane strutture, and by a local Bg fold (which has S3 as axial-plane schistosity). (b) Sketch of a BT fold, i.e. a fold of country-rock schistosity Sa with an axial-plane mylonitic schistosity S,. (c) Sketch of a Bg fold refolded by a BE fold. The latter fold folds S, but also has a weak axial-plane mylonitic schistosity. (d) Sketch of a & fold. These are local intrafolial folds with an axial-plane schistosity S3. (e) Sketch of a Bi fold from the granulite-facies acid gneiss in subarea 7. S is a compositional layering defined partly by bands of magnetite.

pre-mylonite deformations and the extensive effects of mylonitization below the thrust. The dominant schistosity (S’s) is defined by alignment of ellipsoidal quartz and feldspar grains and (001) of micas. It is generally parallel to compositional layering (S) except in a few localities where tight isoclinal folds of S occur with S, axial plane (Fig. 5a). S in these folds has a relict schistosity (S,) parallel to it. A mineral lineation (L,) defined by elongate grains and aggregates lies in S, parallel to 23; fold axes. S, has been deformed by three groups of folds.

Fig. 6. (A) Photograph of a mylonite specimen. Schistosity S, is parallel to the plane of the photograph and the specimen has broken such that two levels of schistosity are revealed. On the upper level L, is bent about the direction of the straight L, on the lower level. Mineral elongation lineation L,, is visible on the upper level parallel to L, on the lower level (see Fig. 6B). (B) Photographic enlargement of a portion of Fig. 6A showing the mineral elongation lineation at an angle in the S,,, plane to the bent intersection lineation 15,.

68 POLES

Fig. 7. Equal-area stereographic projections of field data contoured by the Schmidt method where applicable. Contour intervals are in percent per one percent area. The mylonitic schistosity (S,), myfonitic lineation (I&), and fold axes (BF) prefixes indicate the subarea (i.e. cf. Fig. lb).

Fig. 8. Equal-area stereographic projections of field data contoured by the Schmidt method where applicable. Contour intervals are in percent per one percent area. The country rock schistosity (Sz), lineation (Lz) and fold axes prefixes indicate the subarea (cf. Fig. lb).

POLES

POINTS

Fig. 9. Equal-area stereographic projections of field data contoured by the Schmidt method where applicable. Contour intervals are in percent per one percent area. The country rock schistosity (Sl) and lineation (Lx), mylonitic schistosity (S,) and mylonitic lineation (L,) prefixes indicate the-subarea (cf. Fig. lb). The equal-area projections

306

(1) Broad open folds with no axial-plane structure (B, folds, Fig. 5a) which were rarely observed mesoscopically and which show up mainly as diffuse girdles of S, poles (Fig. 8a, g) that are pervasive throughout subareas 1 and 3. Limited data, the geometric redistribution effects of mylonitization (see below) and the proximity of the p-axes to the average S, plane (cf. Figs. 7d and 8a, g) meant that the mylonite deformation could not be excluded as the cause of these girdles. However since both p-axes lie clear of L, maxima it is considered unlikely that these folds were produced by mylonitization. (2) The mylonitization fold structures BP and Bz described previously. (3) Small sinistral (looking NW) mesoscopic folds Bi (Fig. 5a) which also deform S, (Fig. 5d) and which have a weak retrogressive axial-plane structure (S,) developed in their hinges (defined by alignment of (001) of muscovite) but little to no macroscopic reorientation effects. S, thus has a complex orientation distribution in both subareas 1 and 3 (Fig. 8a and g), even though the effects of Bi can be neglected because of their small scale. The girdle of S, poles in Fig. 8a and 8g is pervasive in each subarea and appears to be due to B, folds, since mylonitization effects are nonpenetrative in these areas and the statistical fold axes are not related to any B!J’ or L, maxima (cp. Figs. 8a, g and 7b, c, h, i; measurements around individual large B, and individual smaller Bg folds also confirm this). The remainder of the S, distribution is attributed to the effects of BT folds and their spread in the plane of S, . L, has also been affected by these folds (Fig. 8b, e, h, j), but since its orientation must have been close to that of B, folds (within 20”) the effects of the latter are minimal and hence its spread is mainly a result of mylonitization. The re~a~i~nsh~~ between subareas 1 and 3 The statistical B, axes in subareas 1 and 3 (Fig. 8a, g) differ markedly in orientation. However, they appear to lie on or close to the average S, plane in subarea 2, the zone of mylonite that separates subareas 1 and 3 (Fig. lb and 7d). When subarea 1 or 3 is rotated about the pole to S, by the requisite amount to align the B, axes, the S, distribution in each subarea is similar. The country-rock blocks in subareas 1 and 3 were presumably continuous across subarea 2 before mylonitization, and this geometry suggests relative ductile rotation of the blocks on the plane of S, during mylonite formation. This is strongly supported by the progressive change in orientation of .&, across subarea 2 by a similar amount and in the same direction as the change in B, (Fig. 4 - this applies whether B, folds are pre-mylonitization or associated with it). DISCUSSION

This discussion considers various aspects of mylonitization, some of which have not previously been emphasized. It also considers the problems of (1)

307

mineral elongation lineation-fold-axes pure shear strain histories in mylonite

parallelism; zones.

and (2) simple

versus

Terminology There is no consensus in the literature for a term or phrase to describe the inhomogeneous form of progressive pure shear. Hence in this paper, it is called ‘inhomogeneous bulk flattening’ as this term best describes the deformation envisaged. Previously it has been called ‘inhomogeneous pure shear’ (Ramsay, 1963, p. 149). This phrase unambiguously describes the deformation but is internally conflicting. Ramsay (1967, p. 484) circumvented this term by using ‘inhomogeneous compressive strain’, but this phrase is slightly enigmatic. Workers on mylonites have sometimes used ‘flattening perpendicular to the lamination’ (Johnson, 1967; McLeish, 1971; Ross, 1973), but the different connotations of flattening (from homogeneous to inhomogeneous) when used in this context make it somewhat ambiguous. Hence these latter phrases have been avoided.

The anastomosing nature of the mybonites Anastomosing foliation seems to be an inherent character of mylonitized rock, especially during early stages in mylonite formation. This could be attributed to initial failure of the country rock along planes of weakness such as old faults, planes of more intense original foliation, weaker (i.e. more ductile) lithologies, and old shears. However, conjugate zones ~~ogous to

POINTS

Fig. 10. Equal-area stereographic projection of mineral elongation lineation (L,,). tour intervals are in percent per one percent area.

Con-

308

the planes of failure in rock undergoing brittle deformation may also occur due to the high strain rate of mylonitization and hence, proximity of deformation to the ductile-brittle transition. The anastomosing character appears to decrease with increase in degree of mylonitization. This accompanies an increase in finite strain and consequently degree of recrystallization seen in thin section (Bell and Etheridge, 1976). Thus in subarea 3, which is dominantly unmylonitized amphibolite-

309

Fig. 11. (A) Photograph and sketch of a BE fold. Note that the mylonitic schistosity folded (a) and that destroying the fold hinge (b) are identical and that the latter appears to have developed from the former as an extension of the limb. Three generations of mylonitic schistosity are apparent-on the L.H. limb. The first (c) is part of the fold but has been cut off by (d) which has in turn been affected by (e). Note that both (d) and (e) are developments of portions of the original fold limb (c) but from opposite directions. (B) Photograph of a BE fold showing change in orientation of the fold axis through greater than 100’ within the axial plane (hammer handle is parallel to fold axes at top of photograph).

310

facies country rock, the thin zones of mylonite within it anastomose considerably, giving a large spread of S, poles into a rough girdle (Fig. 7g). However in subarea 2 very few pods of country rock remain and the dominant mylonitic schistosity is consequently quite planar (Fig. 7d). If mylonitization begins in zones of weakness, the deformation must be inherently progressive. That is, the zones of weakness are initially myionitized and then the country rock adjacent to them is affected and so on. Hence anastomosing early formed mylonitic schistosity which is not paraM to the overall XY plane of the strain ellipsoid for the total myloniteforming deformation event, could be deformed as mylonitization of country rock adjacent to it proceeded (Figs. 11A and 13). The upper and tower contacts of ~y~onite with comity

rock

The top and bottom contacts of the mylonite zone with country rock differ quite markedly. The pseudo~chylite just below the first signs of mylonitization in overlying granulites was produced during or just after the last stages of mylonitization {Bell and Etheridge, in prep~ation). It appears to be a product of late-stage brittle failure due to increase in either strain rate or work hardening on the granulite-facies country-rock side of the mylonite zone (possibly due to its low water content - Bell and Etheridge, 1976). It is considered that the major thrust movements which brought granulite over amphibolite-facies acid gneiss took place at this time, due to the lubricating effects of the pseudotachylite. The great thickness, relative continuity and spatial position (see below) of the pseudotachylite support this. The lack of Sy folds above the pseudotachylite suggests that the mylonitic features there are predominantly ductile-brittle transition effects associated with pseudotachylite formation. During this stage, the deformation would have been almost homogeneous simple shear (see later) and hence no folds would be expected in the country rock above the pseudotachylite. The B;” folds below it formed prior to pseudotachylite formation. The presence of 23; folds (which va.ry in in~nsity with degree of mylonitization) in amphibolite-facies country rock at the sole of the thrust suggest that the mylonite deformation there was a product of inhomogeneous simple shear, in~omogeneous bulk flattening, or some combination of both (see later).

The relationship elongation

between

fold axes (intersection

iineation) and mineral

Three main groups of hypotheses have been postulated to explain the common enigma of parallelism of mineral elongation and fold axes. (1) Mineral elongation develops parallel to fold axes by rotation and rolling in combination with intersecting shear planes (Reusch, 1887; Heim, 1917; Cloos, 1946).

311

(Za) Fold axes develop parallel to mineral elongation by lateral shortening normal to the principal movement, analogous to the lon~tudin~ rolling of corrugated iron (Sander, 1970; Cloos, 1946). (2b) Fold axes form parallel to mineral elongation due to constrictive strains and hence shortening of the Y-axis (Nicholas and Boudier, 1975). (3) Fold axes form at a high angle to the miners-elongation direction and are rotated with increase in strain into parallelism with it (Lindstrom, 1961; Johnson, 1968; Dalziel and Bailey, 1968; Bryant and Reed, 1969; Sanderson, 1973; Harris and Watterson in Powell, 1974). The latter hypothesis can be readily demonstrated to have occurred in this area. A. Pur~l~e~~rnof mineral elon~ut~on and fold axes near the roof and sole of the thrust The dominant country-rock foliations at the roof and sole of the thrust zone (S, and S, respectively) are not parallel (Figs. 4, 9d and Sa, g). However, mineral elongation in S, commonly parallels the inte~ction of S, or S, with S,, The mineral-elongation lineation must lie close or parallel to the long axis (X) of the mylonite deformation strain ellipsoid. If the formation of S,, S, and S, were unrelated tectonic events, then there would be no reason except chance as to why the orientation of X in S, should coincide with the intersection of S, or S, with S, - unless: (1) layer shortening prior to fold nucleation; or (2) passive rotation of fold axes within the plane of S, occurred (Fig. 12). Yet these lineations commonly coincide near the roof and sole of the thrust zone even though S, and S, were not initi~ly parallel. Therefore X could not possibly have lain in both S, and S, at the start of mylonitization and hence the mineral-elongation lineation must have formed initially within S, at some angle to the intersection lineation on at least one side of the mylonite zone. Therefore, the intersection lineation must have passively rotated within S, with increasing strain into parallelism with the elongation lineation on at least one side of the mylonite zone (Fig. 2; cf. Sanderson, 1973; Roberts and Sanderson, 1973; Escher and Watterson, 1974). The effects of homogeneous shortening.. If homogeneous shortening occurred prior to fold nucleation, the earlier foliation plane and the XY plane of the mylonite deformation strain ellipsoid would converge. Rotation would be more rapid towards the X-axis than the Y-axis where X > Y > 2 (Fig. 12). Hence if considerable homogeneous shortening can take place prior to fold nucleation, then the early foliation plane will effectively contain X. However, it will also be very close to containing Y and folds may not nucleate. The large amount of homogeneous shortening in Fig. 12 is unlikely to occur prior to fold nucleation. At lower strains, rotation towards X will be much less and therefore the intersection lineation will lie at some angle to the elongation direction.

Fig. 12. Illustrates the change in simple shear (a) and pure shear ellipsoid with axial ratios 36 : 6 : and planes initially at a high angle rotated towards parallelism with strain ellipsoid in a section normal

orientation of variably oriented lines and a plane during (b) with a strain of 6 corresponding to a deformation 1. N.B. The net effect of either deformation is that lines to the direction of maximum elongation (X) tend to be it. The planes rotate faster towards the XY plane of the to k’ than a section normal to X.

Apparent elongation parallel to the fold axis. If foliation, due in part to aligned triaxial ellipsoid-shaped grains, is preserved though intersected by a more recent metamorphic foliation, there will be within the latter an alignment of the long axes of ellipse-shaped grains parallel to the intersection direction (and hence parallel to any associated fold axes). This is geometrically always the case with the exception of perfectly cigar-shaped grains oriented perpendicular to the intersection. Hence there will be an apparent elongation parallel to the fold axes at low strains during formation of the second foliation. If elongation in the latter foliation was normal to the fold axis, the pre-existing ellipsoidal grains must first become more spherical before they can appear to elongate (provided no recrystallization occurs). The problem which therefore arises is whether mineral elongation parallel to the fold axis is real or apparent. This of course will only be a problem at low strains. It will only be resolvable when the two intersecting foliations differ considerably in character (such as is generally the case in countryrock versus mylonitic schistosities) and then only by using appropriately oriented thin sections. Numerous thin sections were examined in detail with regards to the above problem and it was found that whenever mineral elongation and the intersec-

313

tion lineation were not parallel, the latter did not plunge southwest (see B), [When mineral elongation and the intersection lineation are close to parallel, determination of the orientation of the latter becomes inaccurate due to slight variations in the surface examined from parallelism with S,. The best technique found was to trace a fold axis through a specimen from one profile to another using a distinctive layer.]

L, is distributed throughout the plane of S, (Figs. 4,fiA and 7b, e, h, k; note especially the almost complete girdle in Fig. 7e from subarea 2). However, there is a very strong maximum parallel to the mineral-elongation lineation L,, in each subarea (compare Figs. 7e, k and 10). Figures 6A and B show that where L, is curved witbin the S, plane, it has been rotated towards L,,. Rotation of a large block of country rock (subarea 3) through approximately 90” accompanies the rotation of L, into L,, across subarea 2 (see below). These phenomena suggest that L, formed initially at a high angle to the mineral-elongation direction associated with S, and was then progressively rotated towards L,, due to increase in strain (Fig. 12; cf. Johnson, 1968; Bryant and Reed, 1969; Sanderson, 1973; Roberts and Sanderson, 1973; Escher and Watterson, 1974). This accards well with the discussion in A.

c, fertile rotation of rotationof L,,

large blocks

of ~~~ntr~ rock in ussuci~tiunbitt

Figure 4 shows that in subarea 2, L, remains constant in orientation along the strike of S, but changes progressively in plunge direction from N to WNW to SW across the strike of S, moving to the west. A change in orientation of B, fold axes from subarea 1 to subarea 3, so that they stay on the same small circle about the pole to S, in subarea 2, accompanies this change ‘in L, orientation. This indicates that rotation of L, towards A,, can be relatively homogeneous on a large scale and that rotation of blocks of country rock and hence early fold axes can accompany L, rotation. The rotation itself may be a result of a progressive increase in strain from the sole of the thrust upwards. The uniformity of rotation and the correlation between rotation of L, and B2 and the nSz girdle is quite remarkable and has considerable tee tonic implications.

The mylonitic schistosity (S,) anastomoses around relatively unmylonitized ellipsoidal pods of country rock especially where the degree of mylonitization is low. Mylonitization thus appears to be progressive and heterogeneous. This character may have caused Bg and Bz folds to form concurrently as depicted in Fig. 13, especially since the elliptical nature of the pods

Fig. 13. Diagram (a) shows a section through an elbpsoidal pod of country rock containing schistosity Sz and surrounded by anastomosing mylonitic schistosity (3,) (cf. Fig. 2). Because S, anastomoses around the pod in all three dimensions, any section through the pod at a high angle to S, would look similar to diagram(a). Diagrams (b) and (c) show the possible effects of bulk lnhomogeneous flattening on the pod and its envelope of mylonitic schistosity. Diagrams (d) and (e) show the possible effects of inhomogeneous simple shear in a section containing the mineral elongation lineation. It can be seen that the effects produced vary according to the strain history. For both types of strain history BrCJ and BE folds form simuitaneousfy even though on normal folding criteria they would appear to be of different generations as one apparently folds the scb~stosity-produced axial plane to the other. However for a bulk inhomog~neous flattening strain history (b and c) the 3”, fold axis woufd loop around in its own axial piane through 360’ and close on itself. This would not occur for an inhomogeneous simpleshear strain history and in fact the fold axis should fade out as it swings in its own axial plane towards the mineral elongation lineation. If rotation of fold axes towards the mineral elongation lineation also occurred due ta variation in its intensity, the difference would still be apparent because in the inhomogeneous simple-shear esse the fold axis should not form a closed loop in its own axial p?aae. Apart from this, in the examples shown the fold asymmetry viewed on a cross-section changes in one case but not in the other along the traee of the axial plane. Diagrams (e) and (e) show how the new axial plane mylonitic schistasity forms and destroys the slightly earlier formed myionitic schistosity in a manner similar to that recorded in Fig. 11.

315

caused considerable local strain axes variation relative to that overall. The observed structures (Fig. 11A) are difficult to explain in any other fashion. Deformation such as this could occur readily during inhomogenous simple shear or inhomogenous bulk flattening (see later). In fact BE folds could be used to test which had predominated as the axes would close around the elliptical pods (Fib. 13b, c) with opposite senses normal to L, in inhomogeneous bulk ~at~ning, whereas neither of these relationships should occur in inhomogeneous simple shear (Fig. 13d, e). Progressive simple shear versus progressive pure shear Ductile deformation can be conveniently considered in terms of interaction between progressive pure and simple shear. Natural examples where deformation by one of these has occurred without some contribution from the other are relatively rare (especially in the case of progressive pure shear) and in general ductile deformation involves non-homogeneous forms of both strain types viz., inhomogeneous bulk flattening and inhomogeneous simple shear, respectively. Three main groups of hypotheses can be used to explain the type of deformation involved in the formation of mylonite zones. (1) Mylonite zones are a product of progressive simple shear during or associated with th~sting (Barber, 1965; Bryant and Reed, 1969; Ramsay and Graham, 1970; Escher and Watterson, 1974; Stowe, 1974; Escher et al., 1975; Wilkinson et al., 1975; Bak et al., 1975). (2) Mylonite zones are a product of intensive inhomogenous bulk flattening not necessarily related to any large-scale translative movements (Johnson, 1967; Hossack, 1968; Dalziel and Bailey, 1968; Max and Dhonay, 1971; McLeish, 1971; Ross, 1973; Medlin and Crawford, 1973; Stirewalt and Dunn, 1973; Rousell, 1975). (3) Mylonite zones are a product of some combination of inhomogeneous simple shear and inhomogeneous bulk flattening (Hossack, 1968; Johnson, 1968; McLeish, 1971; Nicolas et al., 1973; Thakur, 1974; Nicolas and Boudier, 1975; Hobbs and Wilkie, 1976). Data from the rocks of the Woodroffe thrust seem to support the latter hypothesis. Fabric evidence The major problem with the in~rpretation of mylonite zones as zones of progressive simple shear has always been the occurrence of apparently orthorhombic c-axes quartz fabrics within them (Johnson, 1967), as such fabrics only develop in metals which have been deformed by progressive pure shear (cf. Williams, 1962 and Dillamore and Roberts, 1965) and theoretically this should also be so for quartz (Lister, 1974). Recently work by Kamb (1972), Hara et al. (1973), Nicolas et al. (1973) and Lister (1974) has shown that although monoclinic fabrics develop theo-

316

retieally and naturally in progressive simple shear, the degree of monoclinicity is often only slight and in such cases reflected mainly in the lower contour intervals and their position relative to 2 (the short axis of the strain ellipsoid). In fact, Hara et al. (1973) tended to regard these fabrics as orthorhombic even though the lower contour levels at high strains (where confidence in their location is high) clearly imparted monoclinicity to the fabric. Hence it is possible that many mylonite fabrics previously regarded as orthorhombi~ are actually mo~~oclinic, Conclusive orthorhombic fabrics do occur in some mylonite zones however, often interspersed with obvious monoclinic fabrics (Christie, 1963 *; Bell and Etheridge, 1976). For this to occur, the deformation must have been inhomogeneous, variable in character from simple or inhomogeneous simple shear to inhomogeneous bulk flattening or almost pure shear-strain histories across the mylonite zone. Fabrics in the Woodroffe thrust are generally monoclinic however (Bell and Etheridge, 1976), which indicates for at least the late stages of penetrative deformation, the dominance of inhomogeneous simple shear over inhomogeneous bulk flattening. The luck of BI;nfolds above the pseudotachylite In the rocks jmmediately overlying the pseudotachylite zone (Fig. 4) there are no Bf folds. Microstructurally however, they contain many ductilebrittle deformation effects associated with mylonitization and pseudotachylite formation (Bell and Etheridge, 1976). Since the pseudotachylite formation post-dated or occurred during the last stages of mylonitization (Bell and Etheridge, in preparation), the deformation during this stage possibly involved dominantly simple shear aided by the lubrication effects of pseudotachylite formation (McKenzie and Brune, 1972; Sibson, 1973). This could therefore have been the time when the major thrust movements which brought granulite-facies over amphibolite-facies rocks, took place. Rotation of L, relative to L,, This can occur by inhomogeneous bulk flattening (Johnson, 1967; Bryant and Reed, 1969; Sanderson, 1973; Johnson, 1968) or inhomogeneous simple shear (Lindstrom, 1961; Harris and Watterson in discussion in Powell, 1974; Figs. 12 and 13b, c), However, if inhomogeneous bulk flattening occurred it should be possible to find examples of L, bent in a continuous loop back onto itself within S,, whereas in inhomogeneous simple shear this would not be expected to occur (Fig. 13). The former situation has not so far been observed and the fold symmetries tend to stay fairly constant (see below). * New grain fabrics are commonly asymmetric when in the same rock the deformed host grain fabrics are symmetric (Bell and Etheridge, 1976). Only the latter fabrics refiect the strain history in such specimens. Hence Riekel and Baker’s (1977) interpretation of X-ray determined fabrics for Christie’s (1963) specimen 62 is not soundly based.

317

Fold morphology ‘The detailed urination of small-scale st~ctures within the context of the larger structural environment can, potentially, supply considerable information on the strain history of particular deformed rocks’ (Hobbs et al., 1976). Figure 4 shows the progressive formation and tightening of Bg folds as S, intensifies across a single outcrop. Since unfolded S, is approximately at right angles to S, and the folds are symmetric, it would appear that the deformation needed to produce them involved inhomogeneous bulk flattening rather than inhomogeneous simple shear. However, the vergence of BF and B,m folds is mainly sinistral though dextral vergences are not uncommon. This may signify a dominant inhomogeneous simple-shear movement of the granulite over amphibolite-facies country rock. Significantly the sense of asymmetry agrees with that of I32 folds (see below). The deformation path From the above discussion it appears that the mylonitization deformation involved inhomogeneous bulk flat~ning and inhomogeneous simple shear. A likely deformation path could have involved initially inhomogeneous bulk flattening with a progressive increase in the inhomogeneous simple-shear component as granulite began to ride over amphibolite-facies country rock. Associated with the increase in inhomogeneous simple shear there may have been an increase in the rate of work hardening (Young et al., 1974, 1975, have shown that work hardening of polycrystalline metals deformed to high strains depends on the mode of deformation). This could have lead to a transition from ductile to brittle defo~ation. Consequent thrust faulting and pseudotachylite production would have involved dominantly simple shear (mylonitization preceding but associated with brittle thrusting has been well documented by Christie, 1963; Johnson, 1960,1967 and MeLeish, 1971). The significance of late stage Bi axes at a high angle of L,, In the Maine thrust zone (Johnson, 1960; Christie, 1963) and the Blue Ridge and Tablerock thrust sheets (Bryant and Reed, 1969), late-stage relatively open folds occur with axes at a high angle to the mineral elongation. They demons~ably refold the isoclinal folds whose axes lie parallel to the mineral elongation. Very similar folds called Bz occur within the Woodroffe thrust zone. They have a constant sinistral vergence and a weak axial-plane structure (S,). The origin of these folds is obscure and has not been discussed to any extent in the literature. Christie regarded them as due to late-stage movements perpendicular to their axes but changed the movement directions by 90° from the earlier mylonitization movement direction to do this. Bryant and Reed appear to believe that they were associated with the last stages of movement on the thrust zones and hence that there had not been sufficient

318

strain after their fo~ation to rotate them into the mineral elongation, Their common occurrence in these major thrust zones is striking and tends to indicate that they were not formed by an entirely separate deformation event but instead are intimately associated with the thrust zone in some way. In the Woodroffe thrust zone they also affect the ~phibolite-facies country rock producing Bi folds, but they do not affect the granuhte-facies country rock. Since mylonitization affected the amphibolite-facies country rock wherever it is exposed (which is mainly adjacent to the mylonite zone), this may not be anomalous. These folds may be associated with late-stage movements on the mylonite zone in the Woodroffe thrust also, because they have the same sense of asymmetry as most of the BT and BE folds (Fig. 4) and their axial-plane structure S, is reminiscent of a mylonitic foliation. CONCLUSIONS (I) Fold axes in the Woodroffe thrust mylonite zone formed initially at a high angle to mineral elongation but passively rotated with increase in strain within their own axial planes until most of them lay parallel to the elongation lineation. (2) This rotation of fold axes was homogeneous in some cases on a macroscopic scale and caused ductile rotation of large blocks of country rock separated by zones of mylonite. (3) Mylonitization is inherently anastomosing, heterogeneous and progressive and hence early formed mylonitie schistosity on the margins of ellipsoidal country-rock pods can redeform (e.g. by folding) since most of it does not lie in the overall XY plane of the mylonite deformation strain ellipsoid. (4) Mylonitization in the Woodroffe thrust may have involved initially inhomogeneous bulk flattening but a progressive increase in the component of inhomogeneous simple shear during deformation resulted in rupture and simple-shear displacement along a thrust plane lubricated by the formation of pseudotachylite. ACKNOWLEDGEMENTS I wish to thank Dr. G. Arnold, Dr. M.A. Etheridge and Prof. B.E. Hobbs for critically reading the manuscript. The field work was conducted while 1 was in a receipt of a Commonwealth Postgraduate Award at Adelaide University. I am indebted to Prof. R.W.R. Rutland for providing departmental facilities and funds for the field work and with Dr. R.L. Oliver, for providing combative argument in the field. REFERENCES

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Progressive deformation and reorientation of fold axes in a ductile mylonite zone: the woodroffe thrust - PDF Free Download (2023)